This invention is generally in the area of Fischer-Tropsch synthesis and, more particularly, relates to an efficient and safe method for transporting products from Fisher-Tropsch synthesis.
Crude oil is a mixture of hydrocarbons when it comes out of the ground. Typically, the mixture is separated into at least three fractions, a gas fraction, an intermediate fraction, and a crude oil fraction, which tend to have some degree of overlap between the fractions. The gas fraction includes mostly C1-3 hydrocarbons, the crude oil fraction includes C4+ hydrocarbons, and the intermediate fraction includes mostly C3-5 hydrocarbons.
Crude oil is often obtained at a location far from where it is ultimately converted into distillate fuel compositions and other products, and is transported to commercial refineries after the gas and intermediate fractions and, optionally, naphtha fractions, have been removed. Crude oil fractions must have a relatively low vapor pressure when they are transported because of safety regulations as well as practical limits on the pumping and storage of volatile crude oil. However, it is common practice to ship crude oil that includes volatile components in concentrations that do not cause the crude oil to exceed the vapor pressure specification or increase the API gravity excessively.
Petroleum-based crude oil, therefore, typically includes C5+ hydrocarbons, with an amount of butane that will not cause the crude oil to exceed the vapor pressure specification. Propane and lighter hydrocarbons are avoided because of their volatility.
The volatility of crude oil in commercial tankers is typically limited to about 9 psia when measured at the shipping temperature. International maritime regulations limit the maximum Reid Vapor Pressure of crude carried aboard conventional tankers to xe2x80x9cbelow atmospheric pressurexe2x80x9d (i.e., less than 14.7 psia). These same regulations limit the closed cup flash point to xe2x80x9cnot to exceed 60xc2x0 C.xe2x80x9d (Safety of Life at Sea (SOLAS) Chapter 22, Regulation 55.1). A practical operational limit is a True Vapor Pressure (not Reid Vapor Pressure) of about 9-10 psia for conventional tankers. A True Vapor Pressure higher than approximately 10 or 11 psia during pumping will make it difficult if not impossible to fully discharge the tanker""s cargo tanks, although the actual pumping performance will depend on the particular ship. Receiving shoreside terminals commonly have a maximum True Vapor Pressure limit of 11 psia, based on the maximum capability of floating roof storage tanks.
Waxy crude oils typically do not contain significant quantities of volatile components and can be shipped at temperatures up to around 1 60xc2x0 F. without exceeding the maximum vapor pressure. Slack waxes from petroleum deoiling and dewaxing operations can also be shipped by tanker in a molten state. These waxes include mostly high molecular weight hydrocarbons and do not typically include significant amounts of volatile light components to cause problems with excessive vapor pressure when the waxes are molten. Accordingly, a preferred method for transporting such waxes is in the molten state.
Like crude oil, natural gas is often located in remote locations. It is often more commercially feasible to convert the natural gas into higher molecular weight hydrocarbons at remote locations than to transport the natural gas to another location for conversion. Many processes, such as Fischer-Tropsch synthesis, can convert methane to higher molecular weight hydrocarbons. Fischer-Tropsch synthesis involves the initial conversion of methane to synthesis gas (xe2x80x9csyngasxe2x80x9d) and the subsequent conversion of syngas to higher molecular weight products. Because it is desirable to limit the amount of processing equipment at remote locations, the Fischer-Tropsch products may be hydroprocessed at commercial refineries far from where the Fischer-Tropsch synthesis is performed.
The products of Fischer-Tropsch synthesis are mostly linear hydrocarbons that often include high melting point wax. A C5+ product stream which is solid at room temperature can be isolated. This product stream is commonly referred to as xe2x80x9csyncrude,xe2x80x9d and can be isolated. In the Mossgas facility in South Africa and Shell""s facility in Malaysia (both developed sites with low to moderate capital costs), methane is converted to Fischer Tropsch syncrude which is refined at the site to finished salable products. When capital costs at remote sites are high, the syncrude could also be transported to commercial refineries for hydroprocessing, for example by hydrocracking, hydroisomerization and hydrotreatment, to provide products with desired properties. This minimizes the construction of expensive facilities at remote sites.
Methods for transporting Fischer-Tropsch derived syncrude from a remote site o a commercial refinery are known in the art (See, for example, U.S. Pat. Nos. 5,968,991; 5,945,459; 5,856,261; 5,856,260 and 5,863,856). One approach has been to isolate a C20-36 syncrude and ship this composition as a solid. A limitation of this approach is that it is difficult and expensive to transport solids, because it requires expensive forming, loading and unloading facilities.
Other approaches have focused on transporting syncrude, or a syncrude which has been partially refined to convert some of the linear hydrocarbons into iso-paraffin and thus generate a syncrude which is liquid at near ambient temperature. One approach to transporting syncrude in the liquid state involves partially dewaxing the syncrude to form a pumpable liquid (See, for example, U.S. Pat. No. 5,292,989). However, this dewaxing may require the construction of facilities which are expensive and difficult to operate in remote locations.
Another approach involves transporting the syncrude as a molten wax. This transportation method does not require the forming, loading and unloading facilities needed to transport solids, or the dewaxing facilities needed to convert the syncrude into a product that is liquid at room temperature. However, Fischer-Tropsch products include a sufficient quantity of volatile hydrocarbons to cause the products to exceed the vapor pressure specifications if the syncrude were shipped at a temperature at which the syncrude is molten.
It would be advantageous to provide an efficient method for manufacturing and transporting syncrude in the liquid state without requiring dewaxing conditions and without exceeding the vapor pressure specifications of the transportation method. The present invention provides such a method.
A novel transportable Fischer-Tropsch liquid syncrude product, and a method for manufacturing and (transporting and/or receiving) the product, is disclosed. These products differ from conventional Fischer-Tropsch liquid syncrude products which contain both volatile and waxy components wherein the mixture has a true vapor pressure in excess of 15 psia when measured at the transportation temperature. The transportable products of the invention have true vapor pressure of less than about 15 psia, preferably less than 11 psia, when measured at its transportation temperature.
The method involves converting a light hydrocarbon feedstock into syngas, converting the syngas to products via Fischer-Tropsch synthesis, and isolating a plurality of transportable products from the Fischer-Tropsch synthesis. The products have true vapor pressures of less than about 15 psia, preferably less than about 11 psia, when measured at their transportation temperature. At least one of the transportable products has a pour point in excess of 20xc2x0 C. At least two of the products are transported separately in liquid form.
The products can be transported using any commonly used means of transportation, including marine tankers, rail cars, pipelines, trucks, barges and combinations thereof. A preferred means of transportation is a marine tanker and, more preferably, each transportable product is either transported in a separate marine tanker or in separate zones within the same tanker. When transported by rail, truck, or barge, each transportable product is preferably transported either in a separate rail, truck or barge, or in separate zones for each transportable product within the same rail, truck or barge.
A C1-2 fraction can also be isolated from the Fischer-Tropsch synthesis and recycled upstream of a syngas generation process, flared, used to produce hydrogen, and/or used for fuel. A C3-enriched fraction, including more than 5% by weight C3, preferably more than 20% by weight C3 and most preferably more than 40% by weight C3 can also be obtained. This fraction can be recycled upstream of a syngas generation process, flared, used for fuel, transported in pressurized tankers, and/or transported in refrigerated tankers.
In a preferred process and business method embodiment, petroleum products such as crude oil can be blended with one or more of the transportable Fischer-Tropsch products, so long as the resulting blended composition has a true vapor pressure of less than about 15 psia, preferably less than 11 psia, when measured at its transportation temperature, and where the Fischer-Tropsch product includes at least 60% by weight, preferably at least 75% by weight, of linear hydrocarbons. The resulting composition preferably has a pour point in excess of 20xc2x0 C. The blending of fractions can occur at any site: the remote Fischer-Tropsch site, a lube manufacturing site, a distillate refinery site, or another location.